1. Field of the Invention
The present invention relates to an image forming apparatus that drives an image carrier for forming a color image on a recording sheet, with a motor.
2. Description of the Related Art
There is an image forming apparatus in which a toner image is formed on a plurality of photosensitive drums used for performing a color image formation, the toner image is transferred onto an intermediate transfer belt, and then, the toner image is transferred onto a recording sheet from the intermediate transfer belt. The photosensitive drum is driven by a motor via a speed reduction gear, so that an angular speed variation or a peripheral speed variation of the photosensitive drum is generated. Therefore, there arises a color misregistration in which toner images of a plurality of colors, which are to be overlaid with each other, are not overlaid with each other during the color image formation, or a banding in which an image, which is to be formed with a uniform density, has a periodical uneven density. For example, the angular speed of the photosensitive drum varies over time as illustrated in FIG. 8A. FIG. 8B is a graph illustrating the variation component of the angular speed, which is obtained by performing Fourier transformation on the angular speed change, for each frequency. In FIG. 8B, peaks appear at about 3 Hz, about 36 Hz, and about 290 Hz. The variation in the relatively low frequency component at and near 3 Hz is an eccentric component of a gear 101, the variation at and near 36 Hz is an uneven rotation of a motor 100, and the variation at and near 290 Hz is a vibration generated when the gear 101 and the motor 100 mesh with each other. The variation in the angular speed at and near 3 Hz causes the color misregistration, and the variation in the angular speed at and near 36 Hz causes the banding.
There has been discussed a technique in which, to reduce the color misregistration, an angular speed of the photosensitive drum is detected to perform a feedback control of a motor, by which the angular speed variation of the frequency component caused by the speed reduction gear is reduced (Japanese Patent Application Laid-Open No. 6-175427).
However, it is difficult to achieve both the reduction in the color misregistration and the reduction in the banding from the reason described below. The angular speed variation illustrated in FIG. 8B can be suppressed by adjusting a feedback gain value, but the angular speed variation of all frequencies cannot be suppressed. According to a sensitivity function in the feedback control, when a variation of a certain frequency is intended to be attenuated, a variation of another frequency is amplified. For example, when a feedback gain, which suppresses the angular speed variation at and near 3 Hz that causes the color misregistration, is set, the angular speed variation at and near 36 Hz that causes the banding is amplified. Accordingly, when the feedback gain is adjusted to suppress the color misregistration, the banding becomes noticeable when a monochrome image is formed.